Glycoproteins are important biomacromolecules that are biosynthesized through posttranslational glycosylation of newly fashioned proteins emerging from the ribosome. Interest in glycoproteins arises at many levels. A long-term goal of the growing field of chemistry-based glycobiology is the delineation of the consequences of glycosylation on critical properties such as protein folding, proteolytic stability, and cell adhesion.1 Such insights could explain why nature bothers to glycosylate otherwise functional proteins. Moreover, glycoproteins have potentially important clinical roles in the context of vaccines, diagnostics, and therapeutics. Indeed, erythropoietin, albeit a heterogeneous glycoprotein,5 is clinically valuable as a treatment for anemia, among other indications.4a 
Many naturally occurring, medicinally important glycoproteins (cf., for example, erythropoietin4a and gp12011) display multiple glycosylation sites containing large oligosaccharide domains. However, given the complexity and variability of biological glycosylation pathways,10 the isolation of homogeneous glycoproteins from natural sources in significant quantity is extremely difficult.
Numerous methods exist for the production of glycopeptides by chemical synthesis. For example, glycans have been introduced into peptides via amino acid “cassettes” with pendant protected saccharides,26-31 through enzymatic manipulations of glycopeptides,32-38 or by conjugation of fully elaborated, complex saccharides to short synthetic peptides.39-41 Larger O-linked glycopeptides have been synthesized using ligation techniques42-43 such as expressed protein ligation.44-46 Bertozzi and coworkers extended the scope of the “cassette” approach by applying native chemical ligation to the synthesis of a biologically active glycoprotein with two single-residue O-linked glycans.47 Tolbert and Wong described the ligation of a 392-residue intein-generated peptide thioester and a dipeptide functionalized with a single N-acetylglucosamine residue. However, none of these approaches has allowed the assembly of complex glypeptides or glycoproteins multiply functionalized (e.g., multiply glycosylated) at designated sites.
Accordingly, there remains a need for novel synthetic methods leading to the preparation of homogeneous polyfunctionalized peptides and/or proteins. Specifically, convergent, stereoselective, versatile methods for preparing such glycopeptides and/or glycoproteins are needed.